This invention relates to a process for the production f multilayer coatings by the wet-in-wet application of an ionically stabilised water-thinnable coating medium to an uncrosslinked, previously electrophoretically deposited lacquer layer, followed by the joint stoving of these lacquer layers.
Industrial lacquer-coating is characterised by efforts to optimise the coating process with regard to its environmental compatibility and energy consumption. Routes to this objective comprise, for example, the use of aqueous lacquer systems and cutting down on energy-intensive process steps, such as reducing the number of stoving steps, for example. In this respect it is customary, in the stoving steps for the individual layers, for the stoving temperatures of subsequent layers to be lower than those of the previous layers in order that the lower lacquer layers are not subjected to too severe a thermal loading.
DE-A-36 06 028 describes a coating process for the production of a two-layer coating, in which after applying a cataphoretically deposited primer coat a second layer is applied wet-in-wet and both layers are thereafter jointly stoved. The material for the covering layer is not water-thinnable; it contains organic solvents and consists of a resin which contains hydroxyl groups, a polyisocyanate compound and additives.
JP-B-91 021 227 describes a three-layer coating with the wet-in-wet application of a water-thinnable stone impact protection layer, which is based on polyester and is cross-linkable with a melamine resin, on to an unhardened electro-dip lacquer layer having a water content of less than 40%.
In this system, one coating layer has to comprise a pulverulent organic resin dispersed in an aqueous vehicle.
US-A-4 537 926 describes water-thinnable stone chip protection materials which can optionally be applied to uncrosslinked electro-dip lacquer layers and which can be stoved jointly with the latter. Copolymers based on unsaturated monomers serve as the basis for the vehicle of the stone impact protection layer. Thermoplastic polymers are described, which means that these stone impact protection materials remain uncrosslinked after being stoved jointly with the electro-dip lacquer.
German Patent Application 41 25 459, which has not yet been laid open to public inspection, describes aqueous coating materials containing special organic polymer powders. These may be applied to various primer coats; it is also mentioned that they may optionally be applied wet-in-wet on to uncrosslinked cataphoretically deposited films. The stoved extender layers obtained give rise to problems of subsequent processability, e.g. during polishing.
DE-A-41 26 476 describes a production process for a multilayer coating comprising the wet-in-wet application of an aqueous, anionically stabilised, amine resin crosslinking coating material based on an optionally urethanised polyester on to a previously cataphoretically deposited layer, followed by joint stoving. The cataphoretically deposited film should undergo a maximum weight loss of 10 weight % on stoving. The substrate can thereafter optionally be provided with a finishing coat. Multilayer lacquer coatings of this type are unsatisfactory as regards their mechanical properties, such as elasticity, resistance to stone impact and the bonding of intermediate layers. The restrictions imposed on the weight loss of the lower cataphoretically deposited lacquer layer are particularly troublesome. However, these are necessary in order that no defects are obtained in the upper layer.